A gag which calls attention to the fact that a show has Loads and Loads of Characters. In response to some summons, event, or strange series of coincidences, the show's entire cast winds up in a single shot, often cramped into an enclosed space. Phonebooths were an early version, followed by elevators, but most modern shows put their own spin on this. The humorous absurdity of such a situation is augmented by the viewer's sudden realization that he recognizes everyone, or the characters trying to fit everyone into a small, small, SMALL setting.
Examples:
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Advertising
The ad campaign for WWE's '08 Royal Rumble involved the more prominent members of its roster rioting in a subway car. Actually, this trope is used a few times in the shows themselves. The aforementioned Royal Rumble may also qualify as an example.
Anime and Manga
An early episode of Maison Ikkoku has the cast spending most of an episode trapped in the bottom of a dried-up well. Why didn't they help each other out, you wonder? The nosy neighbors were just fine partying in the bottom of a well, and the rival suitors didn't want to be the first to leave. After all, the other one would immediately start hitting on Kyoko, right?
Played with in Sailor Moon, where as a part of a plot relevant episode every superpowered girl introduced at this point shows up; the monster is immediately fed up with what's likely a one-sided fight. The same episode lampshades how small the average Japanese house is, as none of the characters can even engage in a fight without tripping over each other.
A later episode has Sailor Moon, The Inner Senshi (Including an Untransformed Minako), Uranus and Neptune, all 3 Sailor Starlights, the Monster/victim of the day, and 2 villains, Sailors Lead Crow and Aluminum Sirein, all in Usagi's kitchen. That's 13 people. The only regulars not there are ChibiUsa and Tuxedo Mask (Both Put on a Bus this season) and Pluto and Saturn. (The former is presumably at home watching the latter.)
Their methods of arrival there are all coincidental as well. Seiya came to visit Usagi to make sure she was alright with her parents and brother away (Calling himself her "Bodyguard", their being Senshi still unknown to each other) the Inner Senshi also stopped by to check on her. Uranus and Neptune's car Broke down outside while, Taiki and Yaten came to get Seyia when a TV crew came by doing door to door dinner interviews (forcing the Starlights, being idols to hide) when the camaraman was attacked by the Villains and turned into a monster.
A NarutoOmake from around the time of the "Search for Tsunade" Arc has nearly every character introduced in the series thus far packed into two scenes: The opening ceremony for a kind of Ninja Olypmics, and a really long line for the restroom that a diarrhea-stricken Naruto needs to use.
The season two opener of Gintama has basically every major character make their way into the Odd Jobs' apartment.
Back in an earlier theatrical short for Anpanman, there was a shot at the end credits showing off every character that had appeared in show since then. However, the cast has expanded to so many characters (and now holds the world record for most named characters in an animated series!), that the amount seems tiny compared to now.
Live Action TV
Done almost once an episode on the American version of The Office. True to this trope's form, we initially knew very few of the supporting cast, but now we recognize everyone, and these conference room scenes show that.
M*A*S*H used this once, with Hawkeye setting the new record for "Most People Crammed Into An Automobile".
This is used as a special attack in Skies Of Arcadia. The captain shouts to attack, and the entire active crew shows up, each using their own abilities to help out.
Least I Could Do spends two or three strips having a bunch of recurring/one-off characters poking their heads in - to note that not one of them minds if John uses the Valentine's Day contest for some petty revenge against Rayne.
One of the earlier running gags in The Simpsons, beginning with most of the town being crammed into Ned Flanders fallout shelter, until the world became fleshed out enough that nearly every potential person was a bystander.
There's also the mob scene in the movie.
Plus the posters. And the intros, too, especially the HD one with fields of people being shown in about two seconds—and yes, a sufficiently devoted fan can name everyone.
In Taz-Mania's Christmas episode, Taz's father narrates the closing of the story mentioning the heartwarming arrival of "all our friends who were sadly cut for screentime" before they arrive en masse for a "heartwarming cast shot."
On several occasions in Harvey Birdman, Attorney at Law, Phil Ken Sebben would call everyone in via the PA system, and they would all be there after the cut. (Plus a few random characters who have nothing to do with anything, like a bear.)
In X-Men Evolution the Professor's study and the control room of the Danger Room (where most the exposition takes place for the show) got progressively more crowded as the show went on.
The The Grim Adventures Of Billy And Mandy episode "Duck!", in which everyone ends up in the same jail cell because of an evil ghost duck, including Hector Con Carne, who protests "I'm not even on this stupid show anymore!"
The final song "Carpe Diem" in the Phineas And Ferb episode "Roller Coaster: The Musical" has every single character which had appeared in the show so far. Incidentally, this highlighted just how weird a cartoon with giant floating baby heads, animal secret agents, and alien bounty hunts is.
